Search results for "Quark"
showing 10 items of 2905 documents
A relation between screening masses and real-time rates
2014
Thermal screening masses related to the conserved vector current are determined for the case that the current carries a non-zero Matsubara frequency, both in a weak-coupling approach and through lattice QCD. We point out that such screening masses are sensitive to the same infrared physics as light-cone real-time rates. In particular, on the perturbative side, the inhomogeneous Schrodinger equation determining screening correlators is shown to have the same general form as the equation implementing LPM resummation for the soft-dilepton and photon production rates from a hot QCD plasma. The static potential appearing in the equation is identical to that whose soft part has been determined up…
Transverse momenta of partons and dimuons in QCD
1978
Abstract Intrinsic (primordial) transverse momenta of quarks and gluons are calculated as well as those arising from recoil (bremsstrahlung) effects, using only the well-known parton distributions as input. The intrinsic kT's lie typically in the range of 150–250 MeV. Recent approaches using heuristic integro-differential equations for kT distributions of partons are shown to disagree with the results obtained by rigorous QCD calculations. The transverse momenta of dimuon pairs produced in pp → μ+μ− + X at the ISR can be solely explained by dynamical recoil effects, i.e., q q →(μ + μ − ) g and gq →(μ + μ − ) q , and no significant intrinsic transverse parton momenta are required. These dimu…
Initial state in heavy ion collisions
2016
Abstract We briefly review advances in understanding the initial stages of a heavy ion collision. In particular the focus is on moving from parametrizing the initial state to calculating its properties from QCD, consistently with the description of hard probes and dilute-dense scattering experiments. Modeling the event-by-event fluctuating nuclear geometry in initial state calculations has significantly improved in recent years. We also discuss prospects of directly seeing effects of particle correlations created in the initial state in the experimental observables.
Up and down quark masses from Finite Energy QCD sum rules to five loops
2008
The up and down quark masses are determined from an optimized QCD Finite Energy Sum Rule (FESR) involving the correlator of axial-vector divergences, to five loop order in Perturbative QCD (PQCD), and including leading non-perturbative QCD and higher order quark mass corrections. This FESR is designed to reduce considerably the systematic uncertainties arising from the (unmeasured) hadronic resonance sector, which in this framework contributes less than 3-4% to the quark mass. This is achieved by introducing an integration kernel in the form of a second degree polynomial, restricted to vanish at the peak of the two lowest lying resonances. The driving hadronic contribution is then the pion …
Determination of the strange-quark mass from QCD pseudoscalar sum rules
1998
A new determination of the strange-quark mass is discussed, based on the two-point function involving the axial-vector current divergences. This Green function is known in perturbative QCD up to order O(alpha_s^3), and up to dimension-six in the non-perturbative domain. The hadronic spectral function is parametrized in terms of the kaon pole, followed by its two radial excitations, and normalized at threshold according to conventional chiral-symmetry. The result of a Laplace transform QCD sum rule analysis of this two-point function is: m_s(1 GeV^2) = 155 pm 25 MeV.
Helicity fractions of W bosons from top quark decays
2008
The helicity fractions of the W boson from top quark decays have been determined by the CDF and D0 collaborations at the Tevatron. It is to be expected that the present experimental errors will decrease with larger data samples and in particular with measurements at the LHC. These improvements have to be matched by advances on the theoretical side. In this presentation we present a progress report on the calculation of the next-to-next-to-leading order QCD corrections to the decay width of the top quark into polarised W bosons.
Determination of the pole and MS¯ masses of the top quark from the tt¯ cross section
2011
We use higher-order quantum chromodynamics calculations to extract the mass of the top quark from the ttbar cross section measured in the lepton+jets channel in ppbar collisions at sqrt(s)=1.96 TeV using 5.3 fb-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider. The extracted top quark pole mass and MSbar mass are compared to the current Tevatron average top quark mass obtained from direct measurements.
Identifying spin and parity of charmonia in flight with lattice QCD
2019
The spectrum of charmonium resonances contains a number of unanticipated states along with several conventional quark-model excitations. The hadrons of different quantum numbers $J^P$ appear in a fairly narrow energy band, where $J^P$ refers to the spin-parity of a hadron at rest. This poses a challenge for Lattice QCD studies of (coupled-channel) meson-meson scattering aimed at the determination of scattering amplitudes and resonance pole positions. A wealth of information for this purpose can be obtained from the lattice spectra in frames with nonzero total momentum. These are particularly dense since hadrons with different $J^P$ contribute to any given lattice irreducible representation.…
Including Tetraquark Operators in the Low-Lying Scalar Meson Sectors in Lattice QCD
2019
Lattice QCD allows us to probe the low-lying hadron spectrum in finite-volume using a basis of single- and multi-hadron interpolating operators. Here we examine the effect of including tetraquark operators on the spectrum in the scalar meson sectors containing the $K_0^*(700)$ ($\kappa$) and the $a_0(980)$ in $N_f = 2 + 1$ QCD, with $m_\pi \approx 230$ MeV. Preliminary results of additional finite-volume states found using tetraquark operators are shown, and possible implications of these states are discussed.
Sivers Asymmetry for the proton and the neutron
2008
A formalism is presented to evaluate the Sivers function in constituent quark models. A non-relativistic reduction of the scheme is performed and applied to the Isgur-Karl model. The results obtained are consistent with a sizable Sivers effect and the signs for the u and d flavor contributions turn out to be opposite. The Burkardt Sum Rule is fulfilled to a large extent. After the estimate of the QCD evolution of the results from the momentum scale of the model to the experimental one, a reasonable agreement with the available data is obtained. A calculation of nuclear effects in the extraction of neutron single spin asymmetries in semi-inclusive deep inelastic scattering off 3He is also de…